The present invention relates to a self-recovering current-limiting device with liquid metal, including electrodes made of solid metal for the connection to an external electric circuit to be protected and several compression spaces which are partially filled with liquid metal.
Soviet Union Patent Publication SU 922 911 A describes such a one-pole self-recovering current-limiting device containing two electrodes made of solid metal which are separated by first insulating bodies which are designed as a pressure-resistant insulating housing. Inside the insulating housing, compression spaces are formed by insulating intermediate walls and second insulating bodies which are arranged therebetween and designed as ring-shaped sealing disks, the compression spaces being partially filled with liquid metal and arranged one behind the other and interconnected via connecting channels of the intermediate walls, the connecting channels being filled with liquid metal and arranged off-center. Thus, in normal operation, a continuous, inner conductive connection exists between the electrodes via the liquid metal. In the current-limiting event, the liquid metal is displaced from the connecting channels as a result of the high current density. In this manner, the electrical connection of the electrodes via the liquid metal is interrupted, resulting in the limiting of the short-circuit current. Subsequent to clearing or eliminating the short circuit, the connecting channels refill with liquid metal whereupon the current-limiting device is operational again. The intermediates walls have to resist the pressure rise during the vaporization of liquid metal and are composed of high-quality ceramic material having a high temperature resistance as well as a high resistance to erosion with respect to the action of an electric arc. In German Patent Application DE 40 12 385 A1, a current-limiting device having only one compression space is described and vacuum, protective gas, or an insulating liquid are mentioned as the medium above the liquid level. According to Soviet Union Patent Publication SU 1 076 981 A, the connecting channels of adjacent intermediate walls are staggered relative to each other for improving the limiting characteristics. It is known from German Patent Application DE 26 52 506 A1 to use gallium alloys, in particular GaInSn alloys in contact devices.
The current-limiting devices according to the related art require considerable constructional outlay and complexity, namely with respect to the great number of parts and to the expensive materials to be used as well as the difficult processing thereof.
Therefore, an object of the present invention-is to provide-a-self-recovering current-limiting device having liquid metal, the device being relatively simple and inexpensive to construct.
The present invention provides a self-recovering current-limiting device including a liquid metal. The device includes a first and a second electrode for connection to an electric circuit to be protected, each of the first and second electrodes being made of a respective solid metal; a plurality of pressure-resistant insulating bodies; and a plurality of insulating intermediate walls supported by-the plurality of insulating bodies, the plurality of insulating intermediate walls and the plurality of pressure-resistant insulating bodies defining a plurality of first compression spaces, the plurality of insulating intermediate walls defining a plurality of connecting channels, the plurality of first compression spaces being disposed one behind the other between the first and second electrodes and being at least partially filled with the liquid metal. The plurality of insulating bodies and the plurality of intermediate walls define a upper half shell and a lower half shell, the upper half shell including a first joining surface and the lower half shell including a second joining surface opposing the first joining surface, the upper and lower half shells being sealingly joined in a region of a common middle plane of the plurality of connecting channels along the first and second joining surfaces. Moreover, the first and second electrodes are each substantially T-shaped, a respective half of each of the first and second electrodes being supported in respective corresponding recess in each of the upper and lower half shells, a respective middle leg of each of the first and second electrodes projecting respectively outward from the upper and lower half shells.
The half shells which are uniformly designed as insulating bodies and intermediate half walls, the sealing connection of the half shells and the bearing arrangement of the T-shaped electrodes give rise to a current-limiting device which is easy and quick to assemble with a few different parts featuring high functional integration, without resulting in disadvantages for the working properties. To produce a range of current-limiting devices having differently scaled rated current ranges is it sufficient to make available these few parts in a manner that their dimensions are correspondingly scaled. The current-limiting device is suitable for two positions of use which are basically rotated relative to each other by 180xc2x0. The frictional connection of the two half shells is effected by known device such as screw connections and/or clamping.
An even number of compression spaces between the electrodes appertaining to a pole, respectively, results in the advantage of designing both half shells identically, involving a further reduction in the number of parts.
An embodiment of the present invention provides a structural combination of several poles appertaining to an electric circuit, the seal along the joining surfaces of the half shells making the seal between the poles at the same time. As opposed to a plurality of single-pole current-limiting devices, a multipole current-limiting device of that kind takes a significantly smaller unit volume and requires less expenditure for assembly.
It is expedient to provide sealing material between the half shells. Proposed for this purpose are, in particular, sealing bodies which are to be advantageously inserted into hollow profiles provided in the joining surfaces for that purpose and/or a sealing layer which is to be applied to at least one of the joining surfaces, for example, by sticking.
A staggered arrangement of the connecting channels of adjacent intermediate walls results in the lengthening of the electric arc, thus increasing the current-limiting effect but can also suppress the formation of a long electric arc across all compression spaces and force the division into a plurality of effective, limiting partial electric arcs. GaInSn alloys as the liquid metal to be used are easy to handle because of their physiological harmlessness. An alloy of 660 parts by weight of gallium, 205 parts by weight of indium, and 135 parts by weight of tin is liquid from 10xc2x0 C. to 2000xc2x0 C. at normal pressure and possesses sufficient electrical conductivity.
In an embodiment of the present invention the half shells feature sleeves in the region of the connecting channels, the sleeves being made of high temperature and erosion resistant insulating material and the rest of the half shells being made of a material which, in comparison, has a lower quality such as cast ceramics. In this manner, the use of high-quality materials is effectively limited to the regions of the half shells which are exposed to the extreme conditions in the short-circuit event. Except for the sleeves, it is advantageous for the half shells to be made of an inexpensive molding material which, in whole, has to satisfy the pressure conditions arising in the limiting event and has to satisfy the low temperature requirements arising outside of the vicinity of the connecting channels.
In another embodiment of the present invention the half shells are made of a non-ceramic material which is resistant to temperature and erosion. In particular, mica is a material which is inexpensive and easy to process, for example using cutting machining techniques, and which has a sufficient resistance to high temperatures and to the action of an electric arc. The half shells can also be manufactured in an inexpensive manner, in particular in the case of great quantities, from a molding material which is resistant to high temperatures or from a glass-ceramics which is easy to mold and to process. In a method for equipping such a current-limiting device the electrodes and a number of connected and therefore easy-to-handle parts of the frozen liquid metal, which number corresponds to the designed number of poles, are inserted into the recesses of the lower half shell which are provided for this. A part which is frozen in such a manner is made of a number of ingots which corresponds to the number of compression spaces of the respective pole and a corresponding number of webs connecting the ingots one behind the other. The part is inserted with its ingots into the compression half spaces formed by the respective half shell, and with its webs into the half connecting channels formed by this half shell. Thereupon, the current-limiting device is completed by sealingly joining the two half shells. The height of the frozen ingots basically corresponds to the later filling level of the liquid metal which is molten at the service temperature. In this context, inlet openings in the insulating body for the liquid metal and outlet openings for gasses to be expelled are not required.
In a method for equipping the current-limiting device according to the present invention the liquid metal is inserted into the compression half spaces formed by the lower half shell in the form of frozen and therefore easy-to-handle unconnected ingots. Thereupon, the current-limiting device is completed by tightly joining the two half shells. The height of the frozen ingots is decisive for the later filling level of the molten liquid metal. Inlet and outlet openings in the insulating body are not required here either.
Subsequent to the above described equipping procedures, the two half shells are expediently to be connected under vacuum or a protective gas. In the so finished current-limiting device, the vacuum or the protective gas constitutes the medium above the liquid level.